1. Field
One embodiment of the present invention relates to a disk drive that uses a disk medium in which servo data is recorded. More particularly, the embodiment relates to a disk drive that has the function of detecting defects in the servo data recorded in a disk medium.
2. Description of the Related Art
In recent years, a servo-writing method, also known as servo-forming method, has come to attract an attention in the field of disk drives, a representative example of which is the hard disk drive. In this method, the servo data for controlling the position of the head is first formed on one side of a stamper and then transferred instantaneously onto a disk medium. The servo-writing method is, for example, either a magnetic transfer method or a method of forming servo data on patterned media.
The servo-writing method, or servo-forming method, is advantageous in many respects over the ordinary method that uses a dedicated device called servo track writer. More specifically, the servo-writing method can write or form servo data with high efficiency. However, the servo-writing method has some problems. The servo-writing method, which employs, for example, magnetic transfer, may erroneously transfer part of the servo data from the master disk to a disk medium, due to foreign matter that has entered the disk drive. A similar problem may also develop in the method of forming servo data on a patterned medium that is a disk medium of discrete-track structure.
Servo data contains address codes and servo-burst patterns. The address codes represent the addresses of the tracks (cylinders) provided on a disk medium. The servo-burst patterns are used to detect the positions in each track. If the servo data is erroneously transferred to a disk medium, some of the servo-burst patterns will not be formed in the disk medium. In other words, some of the servo-burst patterns will become defective. This is a serious problem.
If the servo-data part erroneously transferred exist in the preamble region of the servo data, a synchronous acquisition error will develop and the servo address marks (SAMs) will not be detected. Consequently, wrong servo addresses will be read. Note that the servo address marks constitute a signal pattern that specifies the beginning part of the servo address data. If the servo-data part erroneously transferred exist in the address region of the servo data, the SAMs will inevitably not be detected or wrong servo addresses will inevitably be read. If the servo-data part erroneously transferred exist in the servo-burst region of the servo data, the off-track will not be accurately detected.
The conventional disk drive has the function of detecting servo errors in preparation for reproducing any servo signal read from the head. However, this function cannot serve to detect defects in the servo-burst pattern, which have resulted from the erroneous transfer of the servo data. The servo signal generated from any defective part of the servo-burst pattern is considered to have a normal value though it actually has an erroneous value. As a result, the normal servo operation is performed, inevitably propagating the erroneous value to normal sectors of the disk medium. The erroneous value of the servo signal will result in a head-positioning error over many sectors of the disk medium.
A method of examining a disk medium for erroneous servo-data transfer has been proposed (see, for example, Jpn. Pat. Appln. No. 2003-141837.) In the method proposed, the moderate fluctuation inherent to a disk medium, to which a pattern has been transferred, is distinguished from a partial defect resulting from dust, with respect to the entire circumference of the medium. It is then determined whether any defect has resulted from dust. More specifically, whether an erroneous servo-data transfer has performed or not is determined in accordance with the detachment rate of moving averages, i.e., the ratio of the amplitude for the preamble region (i.e., synchronous burst region) of servo data, to the amplitude of moving averages for 15 servo sectors, of which some preceding the preamble region and the others following the preamble region.
The method described in the preceding paragraph is a technique that is applied to the servo-writing step or the servo-forming step. It cannot be used to examine a disk medium already incorporated in a disk drive, for defects that may exist in the servo-burst pattern recorded in the medium.